This topic came up yesterday from a friend of mine who owns both a Macbook and an iPhone 6. His iPhone is almost out of storage space; about 500MB remaining, and he claims this is due to all the iOS operating systems he has installed over his time with the phone. He also says that this is the same with Apple computers. When he installed up to Mavericks, he lost a lot of storage space because the new operating system "installed separately from the previous one", taking up more space.
It just doesn't make any sense, and I feel like he is wrong, but I'm not sure how to explain it, nor can I find any data to back up his claim or the opposite.
So, the question(s) is, how is storage of the operating systems accomplished for Macbooks and iPhones? Are previous versions of operating systems indeed stored on the phone, even after updating to the newest one? Would this contribute to his lack of available space on the iPhone? Finally, how could this be resolved without taking it to an Apple store?